This invention relates to metallurgical processes, and especially to the construction of a heat treating furnace for annealing coils of metal strip. More particularly, the invention relates to an annealing furnace base that (1) provides thermal insulation between the sealed heat treating compartment and the surrounding atmosphere and (2) supports both the coil or coils of metal strip and a blower wheel and diffuser structure.
In the heat treatment of coiled metal strip, such as steel strip, tightly wound coils of the strip are supported with their axes vertical on an open, gridlike support structure in a sealed compartment, and hot inert gas is circulated through the compartment.
In the annealing process, the steel strip may be heated to as high as 1650.degree. F. Other treatments may involve modifying the chemical composition of the metal. For example, the carbon content of steel strip may be modified by the use of a suitable gas. Furthermore, treatments may involve oxidation, bright annealing, gas alloying, and the application to the surface of materials that will react with the constituents of the metal.
The inert gas used in the annealing process is confined with the coils in a sealed annealing compartment enclosed by a portable inner cover that surrounds the coil or coils with its top and sides and that engages a sealing means at the bottom of the side walls.
The inner cover is surrounded by a portable bell that includes internal heating means or burners to heat the inner cover and the gas therein, and that insulates the inner cover from the surrounding atmosphere outside the furnace. The floor of the sealed compartment is defined by an insulating base that serves both to insulate the sealed compartment from the surrounding external support structure and to support the coil or coils to be annealed, along with a blower wheel and diffuser structure located between the base and the bottom of a coil.
The gas is circulated by a centrifugal blower located in the blower housing and a surrounding diffuser structure directs the hot gas in a somewhat tangential path to the side walls of the compartment where the gas proceeds upward to the top and then downward through the central axial opening in the coils.
It will be apparent that the furnace design must provide a support structure or base capable of supporting great weight particularly in view of the tremendous weight of the coils of steel strip. For this reason, it is desirable to minimize the weight of the furnace components as much as possible.
Many of the furnace components serve primarily an insulating function, and their weight may be minimized by using the lightest possible insulating material that still provides the desired thermal insulating properties. The furnace base, however, must have great strength in order to support the coils, blower housing, and diffuser structure. Accordingly, most lightweight insulating materials would not provide the compressive strength required by the base.
In the past, the furnace base has usually been constructed of steel exterior members to define an enclosure, and the enclosure was filled with a high strength, solid, castable ceramic material. The castable material is by nature quite heavy, and thus inconsistent with the goal of minimizing weight. Also, this base construction resulted in a need for a time-consuming dry-out procedure prior to each annealing operation.
Furthermore, the top plate of the enclosed base in prior art constructions is exposed to great thermal stresses caused by thermal expansion and contraction during the annealing process. These stresses cause fatigue that reduces the effective life of the base and presents a danger of failure that could cause leakage of gas into and out of the sealed annealing compartment.
The annealing furnace base construction of the present invention reduces the difficulties indicated above, and affords other features and advantages heretofore not obtainable.